Conventional EDMs are widely used to accurately machine solid conductive workpieces into molds or a dies. Typically, the workpiece is affixed to a table which is arranged in a work tank, and a copper or graphite tool electrode is attached to a vertically movable quill or ram using a tool holder. The work tank is filled with dielectric fluid such as kerosene, and the tool electrode is positioned extremely close to the workpiece. The space between the workpiece and the tool electrode, known as the work gap, typically ranges in size from on the order of a few μm to a few tens of μm.
If, during a power pulse ‘on’ time, the power pulse is applied across the work gap, the insulation characteristics of the dielectric fluid in the work gap break down and electric discharges occur. At this time, microscopic amounts of the workpiece material are evaporated or become molten due to the heat of the electric discharge, and the liberated material flows into the dielectric fluid. During a power pulse ‘off’ time, the insulation characteristics of the dielectric fluid in the work gap are restored.
As a result of the electric discharges produced during the power pulse ‘on’ time, microscopic crater-shaped holes remain in the surface of the workpiece. Conventional EDMs are equipped with a servomotor which causes the tool electrode to move relative to the workpiece along the Z-axis in order to maintain a constant-sized work gap.
Since it is possible to remove microscopic amounts of material from the workpiece without the tool electrode coming into contact with the workpiece, a cavity having good surface roughness and a shape complimentary to that of the tool electrode may be accurately formed in the workpiece. This type of EDM, known as a sinker EDM, is different from conventional wire EDMs, which uses a moving wire electrode.
During the electric discharge machining process, it is beneficial to remove fragments of the workpiece away from the work gap, to prevent these fragments from causing undesirable secondary discharges. Using a “jump” operation, the tool electrode is moved rapidly up and down along the Z-axis, substantially expelling contaminated dielectric fluid from the gap. In one known example of the jump operation, the tool electrode rises up by at least a depth of the cavity being machined in the workpiece. As a depth of the cavity is increased, however, positive and negative pressures acting on the tool electrode during the jump operation are increased, causing the tool electrode to vibrate and become deformed.
Japanese Patent No. 4-31806 is seen to disclose an EDM which alleviates these types of pressures. As shown in FIG. 1, with this conventional EDM, when the tool electrode is separated from the workpiece at a velocity v2 that is lower than the conventional jump velocity v1, and a distance l between the tool electrode and the workpiece reaches l1, the jump velocity is raised from v2 to v1. Additionally, when the tool electrode is moved from the stroke apex P at velocity v1 in the direction of the workpiece so as to approach the workpiece, and when the distance l reaches l1 the jump velocity v1 is lowered to v2. By reducing the jump velocity at the start and end of the stroke, positive negative pressures are alleviated. Since positive negative pressures vary according to size and shape of the tool electrode and size of the work gap, however, variations of the jump velocity using conventional technologies have the potential to cause lowered machining efficiency. Furthermore, in addition to variations in jump velocity, conventional technologies insufficiently alleviate variations in positive and negative pressures.
As such, it is highly desirable to provide an EDM which overcomes the deficiencies of conventional EDMs. In particular, it is desirable to provide an improved EDM which generates an electric discharge across a fluid-filled work gap formed between the workpiece and a tool electrode, where the tool electrode is caused to rapidly rise up and fall down to substantially expel contaminated fluid from the gap.